Pre-distortion using a by-pass signal

ABSTRACT

The present method disclosed presents a solution to the problem that a fairly large bandwidth has to be associated with the distorted signal used for pre-distortion. According to the present disclosure the method applies different D/A converters for the two signals involved, i.e. for the original digital message signal (A-branch) and the pre-distorted digital signal part (B-branch). In other words, it is suggested to use the same high resolution and moderate sampling frequency D/A converter as used conventionally according to the state-of-the-art, but to use a low resolution with a higher sampling rate D/A for the added pre-distortion signal. The two signals are then combined in their analog domain. Thus, requirements on high sampling rates for both signals at the same time are avoided. Care of course has to be taken to ensure that the two added signals have the proper time alignment or phase alignment

TECHNICAL FIELD

[0001] This invention relates to digital pre-distortion of an inherentlynon-linear device. Specifically it deals with achieving a broadbanddigital signal with high resolution as a pre-distorted signal withoutthe need for fast D/A converters using a high number of bits for highresolution.

BACKGROUND

[0002] One of the operator's main goals is to be able to offer highcapacity to their customers in the network. High capacity in terms ofnumber of channels in a cellular network requires in turn a tighteningof the frequency plan. That is, more frequencies must be made availablein a given area than before. The base station has to handle morecarriers at the same site. Conventional systems like TDMA (DAMPS) andGSM require more channels and upcoming systems like the WCDMA insteadrequires a continuous wide bandwidth. This in turn calls for ultralinear amplifiers.

[0003] Linear amplifiers are used to amplify several carriers at thesame time, as opposite to amplifying each carrier separately and thenadd them up in, for example, a hybrid-combiner. Hybrid-combiners such as90° branch line couplers have the disadvantage that for each doublet ofcarriers there is a 3 dB power loss.

[0004] A linear power amplifier typically has an efficiency of about 6%but it keeps relatively constant efficiency as more carriers are added.Moreover, only one amplifier has to be used for all carriers. The mainproblem with power amplifiers is the linearity of the AM-AMcharacteristics, whereas hybrid combiners do not suffer from this. Mostcellular systems require inter-modulation (IM) products to be in theorder of 70 dB down from the carrier. Extensive work has been done tolinearize power amplifiers of which feed-forward seems to be the mostpromising method. Inter-modulation products are simply subtracted at theoutput of the amplifier by comparing input and output signals of themain amplifier. An error-amplifier adjusts the level of theinter-modulation frequency products (output minus input).

[0005] Feed-forward can improve linearity to a certain degree but thenit becomes very difficult to achieve the last few dB's necessary forfull compliance. A way of further linearizing the amplifier is topre-distort the input signal to the amplifier and compensate for thenon-linearity. There are a number of ways as how to accomplish this. Oneway is to pre-distort within the feed-forward loop of the MCPA itself.Usually this is done in an analog RF fashion. RF pre-distortion (PD) mayalso be done outside the full MCPA.

[0006] Another way is to implement digital pre-distortion. Digital PDmay be used whenever there is a digital combined signal at hand. Theintroduction of so-called software transceivers makes it particularlyconvenient to extract this signal. On a system-level there would be adigital software transceiver, a broadband digital-to-analog converter(DAC), some RF components and the RF MCPA basically connected to theantenna port. A digital pre-distorter would preferably be placed betweenthe software transceiver and the DAC.

[0007] The background to this patent proposal is the need forpre-distortion as a means for or complementing classical linearizationtechniques for non-linear devices. Such devices may be single-carrierpower amplifiers (SCPA) or for example multi-carrier power amplifiers(MCPA), or even passive devices.

[0008] Linearization, as it is usually implemented for broadbandapplications, is to use feed forward techniques. By using this techniqueit is possible to subtract unwanted signal components by comparing thesignal before and after the non-linear device. Linearization can beaccomplished to a certain degree but is normally implemented in analogfashion with its difficulties, which has to be taken care of,essentially at RF frequencies.

[0009] Digital pre-distortion, on the other hand, is usuallyaccomplished at base-band or at some intermediate frequency (IF).Essentially the idea is to perform this where the best control of thesignal is achieved, and also where the carrier frequency is much lowerthan at a real operating frequency.

PRIOR ART

[0010] Designers have been working with linearization basically from thepoint when amplification of electric signals started off. As adescription of the State-of-the-art in linearization techniques one wayis to implement pre-distortion in an analog fashion at analog RFdirectly in front of the non-linear device. Alternatively it may beincorporated within the feed-forward loop in the MCPA itself (if that isthe non-linear device). Some ideas have been put forward to place thepre-distorter also at digital base-band as indicated in FIG. 7. Thedigital signal is copied and fed through a digital pre-distorter andthen added to the original digital signal again before it is fed to theD/A converter. The correction is made completely to the digital signal.

[0011] For instance, in U.S. Pat. No. 5,598,436 is described a digitaltransmission system with pre-distortion. However, the circuitry usesdifferent quantification levels for the phase and the amplitude,respectively.

[0012] Another U.S. Pat. No. 6,172,562 describes a pre-distorter forcontrolling the phase and amplitude in order to linearize a non-lineardevice. The document discusses the problem with high bandwidth demandswhen using high accuracy digital circuits. Separating the phase andamplitude corrections into two parallel branches then solves thisproblem.

[0013] In an article in Electronics Letters Vol. 33 No. 11, 1997 titled“Chip for wide-band digital pre-distortion RF power amplifierlinearization” a custom chip for digital pre-distortion is disclosed inwhich the forward path and the adaptation/control path work withdifferent speeds at a standard resolution of 14 bits. It is pointed outthat the linearizer has to operate with a sampling frequency typicallyfour to eight times higher than the signal bandwidth.

THE PROBLEM

[0014] Thus, the drawback of today's solution is that a fairly largebandwidth has to be associated with the distorted signal. As mostnon-linear devices can be modeled as a power series (see FIG. 2), it isclear that signal components whose frequencies are linear combinationsof the original ones will appear in the output signal. For example, ifthe main non-linearity is a x³-component, there will occur frequenciesthat occupy 3 times as large bandwidth as the original signal (FIG. 1and FIG. 2). And likewise there will occur frequencies fromnon-linearity that have also an x⁵-component, which will give rise toactually 5 times as large bandwidth. The same argumentation can be usedfor further higher components.

[0015] As can be concluded from the above discussion, it is necessary tofeed a signal into the device which in itself has the same requiredbandwidth as the distorted signal wanted to be improved. As therepractically is a relation existing between the dynamic range (number ofbits, or resolution) for a D/A converter and the sampling frequency, itis also clear that achieving a higher bandwidth of the pre-distortedsignal also requires better D/A converters. A low resolution D/Aconverter may operate at a very high sampling rate, but oppositely it isdifficult to design a high resolution D/A converter at the same highsampling rate.

[0016] Another view to the problem is that aliases (periodic copies) inthe spectrum may occur if one is using a too low sampling rate. So,adding a pre-distorted signal to the original one will cause overlappingsignal spectrum as indicated in FIG. 6. The original (analog) signal canno longer be uniquely filtered out properly without suffering fromaliasing effects. Performing a D/A-conversion means essentially that theoriginal spectrum in FIG. 3 should be possible to filter out in thedigital representation as seen in FIG. 4. If the sampling rate fromstart on is too low in comparison to the signal bandwidth, aliasingeffects as shown in FIG. 5 will occur.

SUMMARY OF THE INVENTION

[0017] The solution to the above problem is to use different D/Aconverters for the two signals, i.e. for the original digital messagesignal (A-branch in FIG. 8) and the pre-distorted digital signal part(B-branch in FIG. 8). In other words, we suggest to use the same highresolution and moderate sampling frequency D/A converter as usedconventionally and to the state-of-the-art, but to use a low resolutionwith a higher sampling rate D/A for the added pre-distortion signal. Thetwo signals are then combined in their analog domain. Thus, requirementson high sampling rates for both signals at the same time are avoided.Care should of course be taken to ensure that the two added signals havethe proper time alignment or phase alignment.

[0018] To clarify the above suggestion we may recognize that thedistorted contribution to the signal only has signal components that arein the order of the distortion level that we want to subtract. Forexample, if the distortion in terms of harmonics is 50 dB below thecarrier level on the output of the non-linear device, we only have tosee what resolution we must have to come down to the level we desire.Consider that the desired level is 80 dB below the carrier level, thenit is clear that a dynamic range is needed which is equivalent to(80−50=)30 dB. In terms of number of bits in a D/A converter this equals30/6=5 bit resolution where 6 dB [20 log(2)] accounts for everyadditional bit.

[0019] A method for utilizing a pre-distortion relying on a limitedresolution for the digital pre-distortion part fed to a MCPA is setforth by the independent claim 1, and further embodiments are set forthby the dependent claims 2 to 4.

SHORT DESCRIPTION OF THE DRAWINGS

[0020] The invention, together with further objects and advantagesthereof, may best be understood by making reference to the followingdescription taken together with the accompanying drawings, in which:

[0021]FIG. 1 is an example of input spectrum wherein the bandwidth is Band the amplification is constant, that is G(v)=G₀·v;

[0022]FIG. 2 is an example of an input spectrum together with distortedcomponents as measured at the output of a 3:rd order non-linearity,whereby the bandwidth of the distortion is 3 times larger than theoriginal input spectrum of the signal;

[0023]FIG. 3 is an example of an analog input spectrum represented byits negative as well as positive frequency portions, whereby the low andhigh frequency end points f_(L) and f_(H) respectively indicate thebandwidth;

[0024]FIG. 4 illustrates the analog signal of FIG. 3 sampled at asampling rate f_(S) and the spectrum is repeated periodically withdistance f_(S), whereby the sampling process is repeating both negativeand positive frequency parts;

[0025]FIG. 5 illustrates sampling of a signal with too low samplingrate, whereby the alias effect will destroy the possibility toreconstruct the original analog signal by filtering;

[0026]FIG. 6 illustrates that if sampling rate is too low, the neededextra bandwidth for the pre-distorted signal part will overlap otherduplicate spectra and it is not possible to recover the 3 times (ormore) larger bandwidth signal that is needed for pre-distortion;

[0027]FIG. 7 illustrates conventional digital pre-distortion wherein aportion of the original signal is pre-distorted and again re-insertedinto the signal, which is the D/A converted before being fed to thenon-linear device, the drawback of the solution being the inherent lackof bandwidth of the D/A converter (at the specific sampling rate anddynamic range);

[0028]FIG. 8 illustrates an example of pre-distortion by use of theproposed new method of bypassing to a separate D/A converter with alower resolution, whereby this D/A converter can run at a much highersampling rate with respect to manufacturing standard of today;

[0029]FIG. 9a shows output spectrum from the MCPA without pre-distortionfor an example when a 4-tone carrier ensemble is fed through a nonlineardevice;

[0030]FIG. 9b shows output spectrum for MCPA when the carriersthemselves have been suppressed to highlight the distortion itself;

[0031]FIG. 10a shows a signal spectrum of four equal CW carriers atregular frequency intervals fed through a nonlinear pre-distortiondevice characterized by Equation (2) and fed to the non-linear MCPA asdescribed by Equation (3), whereby the pre-distortion part has beencomputed at a resolution of 3 bits and carriers part at resolution of 15bits;

[0032]FIG. 10b shows a corresponding diagram as FIG. 10a except that afull resolution of 15 bits is used for both signal parts;

[0033]FIG. 11a shows the same spectrum as in FIG. 10a where the carriershave been subtracted to highlight the distorted spectrum part;

[0034]FIG. 11b shows the same spectrum as in FIG. 10b where the carriershave been subtracted to highlight the distorted spectrum part;

[0035]FIG. 12a shows a signal spectrum of four equal CW carriers atregular frequency intervals fed through a nonlinear pre-distortiondevice characterized by Equation (2) and fed to the non-linear MCPA asdescribed by Equation (3), whereby the pre-distortion part has beencomputed at a resolution of 7 bits;

[0036]FIG. 12b shows a corresponding spectrum as 12 a but with fullresolution of 15 bits are used for both signal parts, illustrating analmost negligible difference compared to FIG. 12a;

[0037]FIG. 13a shows the same spectrum as in FIG. 12a except for thecarrier parts, which have been subtracted to highlight the distortedspectrum; and

[0038]FIG. 13b shows the same spectrum as in FIG. 12b except for thecarrier parts, which have been subtracted to highlight the distortedspectrum.

DETAILED DESCRIPTION

[0039] Below will be discussed a method for achieving a broadband signalwith high resolution as a pre-distorted signal without the need for D/Aconverters that have these properties in the same device. The discussionwill utilize spectra for illustrating the present inventive method.

[0040] Assume that we have a signal that is a 4-tone CW signal asdescribed by Equation (1) below. This signal shall be fed through anon-linear device such as for example an MCPA with the non-linearcharacteristics as described by Equation 3. This equation describes apolynomial approximation to the non-linear function and can also beextended to include the phase variation as a function of amplitude. Inorder to counter-act the non-linear behavior of this device apre-distorter is designed to minimize the distortion or inter-modulationproducts in the output of the device. This pre-distorter is found to beas described in Equation (3) below. The design process by which we havefound this particular function and its coefficients is left outside thispatent proposal as it is not considered crucial for the understanding orvalidation of it. Let us just recognize that this particular choice willbe a good example to highlight the process.

[0041] Signal:

a(t):=sin(2·πf1·t)+sin(2·π·f2·t)+sin(2·π·f3·t)+sin(2·π·f4·t)   (1)

[0042] Distorted signal:

α:=0.005

g(t):=a(t)·1+α·a(t)³   (2)

[0043] Pre-distortion signal sent through a MPCA

β:=−0.00005

MPCA(t)=g(t)−α·g(t)³ −β·g(t)⁵   (3)

[0044] If the signal in Equation (1) is fed into the non-linear deviceas exemplified by the MCPA characteristics in Equation (3), we will geta distorted signal spectrum which is depicted in FIG. 9. It should benoted that we have not yet applied the pre-distortion. So we see thatfor this particular choice of non-linear function we have a distortionlevel (or inter-modulation level if you like) that is in this case onlysome 30 dB's below the carrier level.

[0045]FIG. 12 shows the output spectrum of the combined pre-distorterand non-linear device (e.g. MCPA). The two graphs show the comparisonbetween using full resolution (number of bits) to using only for example7 bits for the B-branch, which is the by-passed part. As can be seenthere is almost not a visible difference between the two curves. Byextracting the carrier part it is even more evident by view of FIG. 13.The two graphs show the same signal spectrums as in the previous graph,only the carriers have been taken out and only the distortion is left.It is seen that the inter-modulation for this particular choice ofcoefficients has been improved by around 30 dB.

[0046] As a further example, we may reduce the number of bits in theresolution to only 3 bits (including the sign bit). The result is shownin FIG. 10 together with the same output if we would have fullresolution of 15 bits in both A- and B-branches. Now there is a morenoticeable difference but still, in view of FIG. 11 where the carriershave been removed, the level of distortion/inter-modulation is reducedby 30 dB. The only difference in this case is that a broad band ofspurious signals can be seen that was not there in the previous examplewith 7 bit resolution.

[0047] It may be concluded that according to the simulated results shownin this patent application, the proposed method of using two separateD/A converters for the A- and B-branches will still give very goodresults in terms of intermodulation levels.

[0048] The merit of the proposed invention is that no change in samplingrate needs to be implemented in the original signal in order to providea wide-band pre-distorted signal. A wide-band signal for thepre-distortion part is required to pre-distort a signal on the input ofa non-linear device such as for example an MCPA. However, since thisbroadband signal is added after D/A conversion in the analog signaldomain, there is no restriction on the sampling rate of the originalsignal. Only the by-passed distortion part of the signal has to beimplemented with a high sampling rate. The real benefit from thisproposal is that this by-pass D/A converter need not be a highresolution D/A, but can be any ordinary low cost D/A but with highsampling rate. Thus, digital pre-distortion can be implemented usingexisting D/A converters without loss of performance when connected tothe non-linear device such as the MCPA.

[0049] Linearizing an output signal from an inherently non-linear deviceproducing intermodulation products in its output signal as a response toan input signal, in accordance with the present invention, may beembodied in a numerous number of ways without departure from the scopeand spirit of the present invention, which is defined by the attachedclaims.

1. A method, by means of digital pre-distortion, linearizing outputsignal from an inherently non-linear device producing intermodulation ornon-linear products in its output signal, characterized by the steps of:sampling an original signal with a high number of bits at a givensampling rate for a desired signal resolution of an achieved digitaloriginal signal message; re-sampling the original signal at a lowernumber of bits with a several times higher sampling rate for creating adigital pre-distortion signal to be added to the achieved digitaloriginal signal message, thereby forming a digital pre-distorted signalto be fed to the inherently non-linear device after digital to analogconversion.
 2. The method according to claim 1, characterized by thefurther steps of: using for a first high resolution digital signal asampling frequency corresponding to at least twice the highest frequencyof a desired original signal message bandwidth in accordance to theNyquist theorem; using for a second digital signal a high sampling ratecorresponding to a frequency of at least a desired B-branch errormessage bandwidth, whereby a resolution in bits of the original digitalmessage is defined by a desired dynamic range below a carrier indecibels level being A_(DB), and the resolution in number of bits of thepre-distorted digital signal to be added is defined as at least(A_(DB)−B_(DB))/6, wherein B_(DB) is the level of harmonics below thecarrier in decibels to be cancelled.
 3. The method according to claim 2,characterized by the further steps of: selecting for the first highresolution digital signal A/D conversion of 16 bits, and selecting forthe second low resolution digital signal A/D conversion of 8 bits with,e.g., three times the sampling frequency of the first high resolutiondigital signal.
 4. The method according to claim 2, characterized by thefurther steps of: selecting for the first high resolution digital signalA/D conversion of 16 bits, and selecting for the second low resolutiondigital signal A/D conversion of 6 bits at an even higher sampling e.g.with five times the sampling frequency of the first high resolutiondigital signal.